Method of heating parison preforms



Dec. 24, 1968 Filed Aug. .7, 1967 c. 1.. SEEFLUTH METHOD OF HEATINGPARISON PREFORMS ,2 Sheets-Sheet 1 INVENTOR.

C..L. SEEFLUTH A TTORNEYS Dec. 24, 1968 c. L. SEEFLUTH 3,413,393

IETHOD OF HEATING PARISON PREFORMS Filed Aug. 7, 1967 v 2 Sheets-Sheet 2Iikn ' x. l' l l e FIG. 3d

ENTOR INV C.L. SEEFLUTH rronusr: r

United States Patent 3,418,398 METHOD OF HEATING PARISON PREFORMSCharles L. Seefluth, Bartlesville, Okla., assignor to Phillips PetroleumCompany, a corporation of Delaware Continuation-impart of applicationSer. No. 437,175, Mar. 4, 1965. This application Aug. 7, 1967, Ser. No.658,685

6 Claims. (Cl. 263-52) ABSTRACT OF THE DISCLOSURE A hollow parison usedfor blow molding hollow articles is heated by placing the parison into ahigh heat conductivity mold having a flexible, low heat conductivityliner, heating the mold to a temperature suflicient to heat the parisonto a temperature below the crystalline melt point of the parison, andremoving the parison from the mold. In a preferred embodiment, fluidpressure is applied between the mold and the liner after the parisonreaches the desired temperature, the liner then pushing the parison fromthe mold so that the liner is peeled from the parison.

Cross references to related applications This is a continuation-in-partof application Ser. No. 437,175, filed Mar. 4, 1965, now U.S. Patent No.3,347,- 966.

Background of invention This invention relate-s to heating a parison.

Blow molded objects can be formed in two ways. One method is to extrudea parison, place the hot parison into an article mold and blow theparison in the mold. Another method is to extrude a parison of aparticular shape, let the parison cool, reheat to a desired temperature(which, if oriented articles are desired, is a temperature slightlybelow the crystalline melt point of the polymer), place the reheatparison in an article mold and blow the parison. In the latter method,and particularly if oriented articles are desired, it' is necessary touniformly heat the parison so that the expansion takes place uniformlywhen the parison is blown. It is also necessary to remove the parisoncarefully so that no distortion of the parison takes place before theblowing operation.

A problem encountered in blow molding of thermoplastic articles, inparticular oriented thermoplastic articles such as oriented bottles, isdistributing the materials so that the finished article has the wallthickness which is essentially the same throughout. Variations in wallthickness may be caused by a portion of the parison being heated to atemperature which is just slightly dilierent from that of the remainderof the parison; this slight difference in temperature in a thermoplasticmaterial at just below its crystalline melting point is sufficient tocause uneven blowing of the parison.

Summary of invention It is an object of this invention to provide amethod for heating a hollow thermoplastic parison.

According to the invention, a parison having a closed end is heated to atemperature just below the crystalline melting point of the parisonmaterial in a mold which has a high thermal conductivity and which has aflexible liner made of a material which has a thermal conductivity whichis low relative to that of the mold.

Patented Dec. 24, 1968 Brief description of the drawings In thedrawings, forming a part hereof, in which like reference charactersindicate like parts in the various views, FIGURE 1 is a cross sectionthrough a parison; FIGURE 2a is a cross section through a heatingapparatus showing heating of the parison; FIGURE 2b is a cross sectionthrough the heating apparatus showing the method of removal of theheated parison; and FIGURES 3a, 3b, 3c, and 3d are cross-sectional viewsthrough a mold showing the various stages of blowing the parison.

Description of the preferred embodimen s Preformed parisons can beheated in accordance with the instant invention to any temperature up tothe crystalline melting point. Generally the parisons will be heated toa temperature from about 1 to about 50 degrees F. below the crystallinemelting point. At this temperature the plastic undergoes maximumorientation on stretching to yield a product having greatly increasedstrength and clarity.

The crystalline melt point of polymers such as polypropylene andpolyethylene can be determined by heating a small piece of the plastic(usually film) between cross polaroids under a microscope equipped withmeans for heating the polymer. The specimen is heated slowly and thecrystalline melt point is the temperature at which birefringencedisappears.

The flexible liner which lines the inner surface of the mold can be madeof rubber (including silicone rubber) or any flexible material which hasa substantially lower heat conductivity than that of the mold. It isnecessary that the heat conductivity of the liner which is in contactwith the parison being heated be lower than that of the mold so that theliner can act as a buffer and thus bring about more uniform heating ofthe parison. The effect of any hot spots in the mold will be reduced oreliminated because of the presence of the low heat conductivity linerwhich retards the direct transfer of heat inward toward the parison andallows more lateral dissipation of the heat. Thus the heat istransferred inwardly and then laterally through the mold. In a preferredembodiment the heat conductivity of the liner while substantially lowerthan that of the mold is higher than that of the material comprising theparison. Thus, while the heat is transferred generally inwardly throughthe liner and into the parison, if the parison has a thick spot or aspot which for any reason absorbs a greater amount of heat, a portion ofthe heat can be conducted laterally along the flexible liner to thisarea which is absorbing more heat. For instance, in heating a parisonwhich has a wall thickness at the closed end which is less than that ofthe rest of the parison, heat can be conducted along the flexible linertoward the area of greater thickness of the parison where more teat isbeing abnorbed. Also, with a liner having a heat conductivity higherthan that of the parison it is possible to supply heat to the parison asquickly as it is absorbed. Thus this arrangement makes possible theoptimum in quick, uniform heating of the parison.

The mold can be formed of any suitable high heat conductivity material,such as aluminum, carbon steels, stainless steels, and copper.

Referring now to FIGURE 1, a parison 1 of thermoplastic material isshown having a closed end 2 and an open end. As previously mentioned,the parison can be composed of any thermoplastic material, such aspolyethylene, polypropylene, polybutene and copolymers thereof. Thethickness of the closed end 2, T, is less than that of the thickness ofthe walls, T.

Referring now to FJGURES 2a and 2b, mold 3 of high heat conductivity,having a flexible liner 4, defines a shape conforming to the shape of aparison 1 suitable for heating parison 1. Mold 3 is provided withelectrical resistance elements 5 to provide means for heating the moldto a suitable temperature. Other heating means, such as a circulatingfluid of heated material, can be provided instead of the resistancewires 5. A conduit 6 is connected to an air hose 7 which is adapted toprovide means for fluid pressure to be introduced between the flexibleliner 4 and the mold 3. In operation, after the parison 1 has beenheated to a suitable temperature, fluid pressure is introduced throughline 7 and conduit 6 to push the parison out of the mold. As can be seenin FIGURE 2b, the flexible liner 4 is peeled from the parison 1 as theparison is stripped from the mold. The fluid pressure in the area 8pushes the flexible liner upwardly and, thus, peels the flexible linerfrom the parison 1.

Referring now to FIGURE 3a, mold halves 10 and 12 form a suitable moldcavity 11 for a blow molded thermoplastic article. A mandrel 13 holdsparison 1 and introduces fluid pressure into the interior 14 of theparison.

In operation, a parison 1 having a thinned end portion 2 is held onmandrel 13 and introduced into mold cavity 11. A differential pressureis introduced between parison interior 14 and mold cavity 11, causingthe parison to expand. This pressure differential can be introduced byblow molding or vacuum molding. In the case of vacuum molding, suitablevacuum ports (not shown) can be included in the walls of mold halves 10and 12. The first step of the blowing operation is shown in FIGURE 3]).As the parison 1 begins to expand, the thinned portion 2 expands firstand comes into contact with the bottom of the mold cavity. Expansion ofthe parison 1 then takes place at areas adjacent to those areas incontact with the mold cavity. Thus, the area of thinned wall thicknessexpands first and those areas adjacent that area of thinned wall sectionexpand next. As shown in FIGURE 30, the mandrel 13 is withdrawn from themold as blowing progresses. It can be seen from FIGURE that the sides ofthe article are formed sequentially from the bottom to the top. Thistype of blowing operation produces a highly oriented thermoplasticarticle. The completely blown article is shown in FIGURE 3a. In FIGURE3d, the mandrel 13 is completely withdrawn from the mold cavity and theparison 1 has been completely expanded to fill the mold cavity.

Example A parison 3 inches long having one closed hemispherical end .594inch in outside diameter and 0.065 inch thick and one open end .838 inchin outside diameter and 0.057 inch thick, the wall thickness taperingfrom the open end to a section .111 inch thick about 0.380 inch from theopen end then tapering to the hemispherical end section, and the outsidesurface smoothly tapering, similar in appearance to that of FIGURE 1,was produced by conventional injection molding from polypropylene havinga density of 0.905 gram per cc. and a melt flow (ASTM D1238- 57TCondition L) of 6.5.

This parison was inserted into a heating device similar to that ofFIGURE 2 having a block temperature of 335 F. and was heated for about3.5 minutes. Air at about 5 psi. pressure was introduced between theblock and the liner thus forcing the parison upward and onto a blowingand thread forming mandrel where the threads were formed by clamping andpressing the neck mold member firmly together about the first 0.5 inchnear the top of the parison.

The mandrel held parison, in the vertical position with the threadsuppermost, was then enclosed in a 7 ounce bottle mold (a mold for abottle which has a 7 ounce water capacity) and positioned near thebottom. Air under pressure was admitted to the inside of the parison andsimultaneously the mandrel was raised with respect to the mold at a ratecorresponding to the development of the blowing parison so as to come tothe uppermost position within the mold in about the same time it takesthe parison to fully develop into a blown bottle. This occurred in about6 seconds. Two additional seconds were allowed for the mold to cool thebottle, after which the mold was opened and the bottle removed. Theresulting bottle was filled with water, capped, and brought to atemperature of 35 F. and dropped three times onto a concrete surfacefrom a height of 8 feet without breaking or subsequently leaking.

Another such bottle produced from the same polymer and in the samemanner as the first was cut into tensile specimens. The wall samplesshowed tensile strengths varying between 10,000 and 20,000 pounds persquare inch as compared to 5,000 to 6,000 p.s.i. for unorientedpolypropylene.

Other bottles made by this same process from this same polymer werejudged by several experts in the field of plastic packaging to haveexceptional clarity for an unmodified polypropylene bottle.

Samples taken from bottles made by conventional blow molding of the samepolymer exhibit tensile strength in the range of unoriented polymer, andsuch bottles break when filled with water, capped, brought to atemperature of 35 F. and dropped onto a concrete surface from a heightof 8 feet.

While this invention has been described in detail for the purpose ofillustration it is not to be construed as limited thereby, but isintended to cover all changes and modifications within the spirit andscope thereof.

I claim:

1. A method of uniformly heating a hollow thermoplastic parisoncomprising:

placing said parison in contact with a flexible liner within a mold,said mold having a higher heat conductivity than said liner;

heating said mold and transferring said heat through said liner intosaid parison until said parison is heated to a temperature just belowthe crystalline melting point of said thermoplastic material comprisingsaid parison; and

removing said parison from said mold.

2. The method according to claim 1 wherein said liner has a higher heatconductivity than said parison so as to transfer heat to areas of theparison having a greater capacity for absorbing heat.

3. The method according to claim 2 wherein fluid pressure is appliedbetween said mold and said liner after said parison reaches the desiredtemperature and said liner pushes said parison from said mold so thatsaid liner is peeled from said parison.

4. A method of uniformly heating a hollow thermoplastic parisoncomprising:

placing said parison in contact with a flexible liner within a mold;

heating said mold;

transferring heat within said mold inwardly toward said liner andlaterally through said mold along substantially the interface of saidliner and said mold; transferring heat inwardly into said liner;

thereafter transferring a portion of said heat on in wardly into saidparison;

transferring another portion of said heat laterally through said linerto points of greater heat absorption on said parison so as to heat saidparison to a temperature just below the crystalline melting point ofsaid thermoplastic material; and removing said parison from said mold.

5. A method according to claim 4 wherein fluid pressure is appliedbetween said mold and said liner after said parison reaches the desiredtemperature and said liner 5 6 pushes said parison from said mold sothat said liner is crystalline melting point of said thermoplasticmapeeled from said parison. terial comprising said parison; and

6. A method of uniformly heating a hollow thermoplasremoving saidparison from said mold,

tic parison comprising:

References Cited placing said parison 1n contact with a flexlble liner 0"I" Within a mold, said mold having a higher heat con- UNITED SlATESPATENTS ducfivity than said liner; 2,181,821 11/1939 S6315 2635 heatingsaid mold and transferring said heat through 2,213,902 9/1940 Damassaidliner into said parison until said parison is heat- 2,779,578 1/1957Corey 2635 ed to a temperature of 1 to 50 degrees F. below the 10 JOHNJ. CAMBY, Acting Primwy Examiner.

